The contrast in an X-ray image is usually caused by different attenuation properties of the substances relative to the X-radiation produced by an X-ray machine. When examining a patient it is possible on the basis of the different attenuation properties of bone tissue and soft part tissue to analyze the structure of the bone in the body interior of a patient on the basis of the contrast. Organs or vessels having an attenuation property similar to the surroundings cannot be examined in a conventional way because of too low a contrast.
For this reason, a contrast agent is used when examining an organ supplied with blood, for example, a heart or a liver. The contrast agent exhibits a different attenuation property by comparison with the surrounding tissue and so a visible contrast is produced between the organ and the surroundings in the image.
The propagation of the contrast agent in the patient's body is a highly dynamic process. After a certain time, the concentration of the contrast agent in an examination region being viewed rises steeply at first, reaches a maximum and subsequently falls back again. During a main examination the scanning of the examination region is not started immediately, but only after a delay time after the instant of contrast agent administration, such that the examination region has a high concentration of the contrast agent during scanning. The delay time is determined at the beginning of each examination by means of a preliminary examination in which a slight quantity of the contrast agent is fed to the patient, and in which attenuation values at consecutive instants are acquired as a measure of the concentration of the contrast agent at the scanning position. The delay time results in this case from the time interval between the contrast agent administration and the observed maximum in the attenuation values.